The present invention relates to a hand clamp, more specifically to one that provides adjustability, by reason of being able to change the compression of a spring, and also provides compound action. By compound action is meant that this clamp utilizes a coiled compression spring which transmits its adjustable force to the clamp arms through connecting, movable links, instead of a torsion spring applying its closing forces directly to the arms.
A variety of hand clamps have been known heretofore as, for example, those described in U.S. Pat. Nos. 2,001,707; design patent 138,285; 2,815,777; and 3,779,108.
The device described in U.S. Pat. No. 2,001,707 is referred to as a hand tool of the plier type having provision for releasably holding the retaining cone-shaped split keepers for the valve spring retainer disks of internal combustion engines or the like. Accordingly, it is a quite specialized tool. However, it does provide the general feature of a spring which is coiled about a pivot pin, and about which the plier handles are pivotable; the spring has extensions that engage with the respective handles so that the plier jaws are urged toward each other into a closed position.
U.S. Pat. No. 2,815,777 discloses a spring actuated miter clamp similar in many respects to the aforesaid U.S. Pat. No. 2,001,707. However, for its own special purposes, it has pivotable jaws mounted on the closing ends of the lever arms or handles.
U.S. Pat. Nos. 3,779,108 and design patent 138,285 also disclose clamps of different configurations including spring members.
A major difficulty that arises in the use of spring clamps and the like, as described in the references cited above, is that although only a moderate force is necessary initially to separate the clamp jaws or jaw portions, a substantially greater force is required to cause the clamp jaws to be opened a substantial distance. For example, an opening of approximately two inches is necessary so that a fairly large workpiece can be grasped or clamped. As a consequence, conventional torsional spring clamps are unsatisfactory for use by persons having limited hand strength. Additionally, if one manages to get the clamp open, the clamping force may be much greater than needed for the job to be done. Further, there is no way to control this force.
Accordingly, a primary object of the present invention is to overcome the aforesaid difficulty and to provide a clamp construction such that, for a given spring adjustment, only a slightly greater force is required to cause the jaws to be spaced apart a greater distance, compared with the force used initially to separate the jaws.
Another object is to enable ready adjustment of the spring force to be applied to a workpiece by the clamp, and to preserve substantial constancy in required gripping force for the two operations noted as the value of the spring force employed is varied. Reference to actual tests performed will make the above object clear.